Because of rapid expansion of mobile traffic and video services, there is a demand for increasing a communication capacity in a core network. The demand for increasing the capacity tends to continue in the future. In order to increase the communication capacity continuously at a limited cost, it is effective to improve usage efficiency of the network by efficiently operating resources of the network.
Especially in an optical communication network that handles a significantly large amount of information, it is important to use optical frequency bands efficiently that are communication resources. If an optical frequency band in an optical communication network is used, it is necessary to consider deterioration in optical signal quality caused by constraints of various physical laws in optical signal transmission. The physical constraints in this case include a crosstalk between adjacent wavelength channels in the wavelength multiplexing optical signal transmission, deterioration in an S/N (Signal/Noise) ratio caused by an optical fiber loss or an optical noise added by an optical amplifier, for example. In addition, the above-mentioned physical constraints also include the passband narrowing effect caused by passing through a plurality of optical band pass filters (BPF). Considering and dealing with these physical constraints make it possible to improve the usage efficiency of the resources in an optical communication network. As a result, the transfer cost of large volumes of information bits can be reduced.
Patent Literature 1 discloses an example of the technologies to control the deterioration of received signal quality caused by passing through a plurality of optical band pass filters (BPF) as mentioned above.
In the method of setting a passband of a path described in Patent Literature 1, a wide passband is set in a wavelength selective switch through which the path passes, with respect to a path that passes through a large number of wavelength selective switches, whose filtering penalty becomes large. With respect to a path that passes through a small number of wavelength selective switches, a narrow passband is set in the wavelength selective switches through which the path passes. A path with a narrow passband is arranged adjacent to a path that requires a wide passband.
It is said that the above-described configuration makes it possible to provide technologies to construct an optical transmission network in which reception quality of signal light on each path is improved as a whole without limiting a transmission rate or a scale of the optical transmission network as far as possible.
It is described in Patent Literature 2 that a bandwidth variable communication system uses a higher order modulation format and a narrowband filter corresponding to it for an optical communication path having a short transmission distance, and a lower order modulation format and a broadband filter corresponding to it for an optical communication path having a long transmission distance. It is said that this system makes it possible to reduce a spectral range required in total and improve the frequency usage efficiency.